The present invention relates generally to a means and a method for processing semiconductor and other materials, and more particularly, to a means and a method for monitoring the change in thickness of a thin material layer during a thickness-changing operation.
There are a variety of apparatus and methods available for changing the thickness of a given material, including deposition techniques for increasing the thickness from a zero base to some desired thickness, and etching and ion-milling techniques for reducing the thickness to some desired level or completely removing the material at a particular location. In semiconductor device processing, the ability to monitor the thickness during such a thickness-changing operation is frequently critical because of the very small dimensions and tolerances involved. Such monitoring is essential in order to control and/or stop the thickness-changing process at a desired thickness of the material.
For example, with reference to material thickness reduction, reactive ion etching or plasma etching is now typically used to delineate fine line patterns and trenches in thin films of either insulators, semiconductors, or metals by means of the removal of portions of these films in a plasma discharge. The wafer to be etched is placed in a plasma chamber into which a gas mixture is directed at a reduced pressure. In the presence of a plasma discharge generated by the application of RF energy, reactive species are generated from the feed gas by processes such as dissociative or impact ionization. Portions of the thin film are removed by chemical reaction between the reactive species and the film, as well as by bombardment of the film by ions present in the plasma. The gaseous reaction products formed by the chemical reactions and the ion bombardment of the film are continuously removed from the chamber using a vacuum pump.
One of the problems encountered in the use of reactive ion etching is the insufficient reproducibility of the etching rate. In part, this etch reproducibility problem is caused by variations in the plasma composition due to the time dependent presence of etch products, difficulties in completely controlling the surface temperature of the wafer or wafers to be etched, and batch-to-batch variation in the quantity of material to be etched, or the load. Because of this variation in the etching rate, reactive ion etching requires monitoring to detect the completion of the etching process. In this regard, it is important to detect the end of the etching process in order to terminate the etch before over-etching occurs in the sublayer below the layer being etched. Such over-etching is detrimental not only because it attacks the substrate or sublayer below the layer being etched, but also because it causes undercutting of the etch pattern, thereby altering the dimensions of the desired features in the etched layer.
In one endpoint detection scheme, a majority chemical species from the layer being etched enters the etching plasma and is observed by monitoring a relevant spectral line for that majority species as the etching process consumes the layer being etched. The time to terminate the process is inferred from changes in the intensity of this monitored majority species spectral line. However, when the composition of the etched layer and its sublayer therebelow are similar or the same, then monitoring of the majority species from the etched layer will not provide a determination of the etch endpoint. A similar problem is encountered when techniques are utilized to compensate for etch loading non-uniformities. For example, an aluminum film on a wafer is many times etched by disposing the wafer on a high purity aluminum target to thereby prevent a sudden large excess of etching species near the end of the etch process which would cause an attendant undercutting of the aluminum film. However, the use of this aluminum target prevents the determination of the etch endpoint by monitoring the majority aluminum species.
This problem is especially acute for the etching of GaAs down to a layer of AlGaAs. Optical emission or laser induced fluorescence of atomic aluminum present in the etching plasma is not sensitive enough to detect the subtle aluminum concentration change as the interface between the GaAs and AlGaAs layers is reached, because most RIE reactor chambers are made of aluminum. The aluminum sputtered from the chamber walls during the RIE process essentially buries any such aluminum concentration change.
Additionally, the above-described prior art schemes are restricted to endpoint detection. They cannot be used to stop an etch process with a predetermined thickness remaining of the layer being etched, without resorting to etch timing, with its attendant inaccuracies.
Likewise, with reference to thickness increasing processes such as deposition, applicant is not aware of any monitoring apparatus or techniques which can be used to stop a deposition process after a thin layer of a first material has been deposited over a second material, without resorting to some form of deposition timing or wafer weighing step.
The invention as claimed is intended to remedy the above-described drawbacks in thickness-changing process monitoring.
The advantage offered by the present invention is that it allows accurate monitoring of a thickness-changing process. The invention permits the monitoring of the deposition of a very thin layer of material over a second different material. The invention permits the monitoring of material thickness reduction down to a very thin layer above a second different material. Finally, the invention permits the accurate monitoring of the etch-through from a first material to a second different material.